Asphalt base solid composite propellants



Sept. 19, 1961 H. w. MACE 3,000,717

ASPHALT BASE SOLID COMPOSITE PROPELLANTS Filed-Dec. 12, 1945 INVENTOR.

HARRY M. MACE %&M/4%M A TTORNEYS 3,000,717 I ASPHALT BASE SOLID COMPOSITE PROPELLANTS Harry W. Mace, Glendale, Califi, assignor, by mesne assignments, to Aerojet-General Corporation, Cincinnati, Ohio, a corporation of Ohio Filed Dec. 12, 1945, Ser. No. 634,609 9 Claims. (Cl. 52.5)

This invention is concerned with asphaltic base propellants, and particularly with solid or plastic propellants of this type for use in jet propulsion motors. The invention provides improvements to the end that such vpropellants resist sagging and cracking-over a wide range of temperatures.

This application is a continuation-in-part of my application Serial No. 565,373, filed November 27, 1944.

It has been proposed heretofore 'to use propellants for rockets or jet propulsion motors which are solid or plastic mixtures of asphaltic base having disbursed therein a finely-divided inorganic salt which serves as the oxidizer for the asphalt and other organic materials in the propellant. Such mixtures are self-combustible, and are burned to produce gaseous combustion products that are exhausted through an exhaust nozzle from a combustion chamber at high velocity, thereby producing the desired propulsive thrust. Rockets and jet motors may be subjected to a wide range of temperature before the propellant is burned, for they may be stored or employed on aircraft under tropical conditions and at the low temperatures of high altitudes. Asphaltic base charges of the type described frequently have been unsatisfactory, because they tend to "flow-and sag 'at hi'gh temperatures, say of the order of 150 E, and to become excessively brittle and to crack at low temperatures of 30 F. The sagging or cracking of the charge changes the surface area exposed to burning.

This in turn changes the burning characteristics of the charge and may create uncontrollable pressures during combustion with resultant blowouts.

I have discovered that the undesirable cracking at low temperatures may be inhibited and, in large part, prevented in plastic asphaltic base propellants (containing as oxidizer an intimately incorporated, finely-divided inorganic salt, such for example,as a chlofa'te, perchlorate, chromate, or permanganate) by incorporating in the charge substantial proportions of a condensation product of sebacic acid and a polyhydric alcohol.

The proportions of the condensation product to be employed are not precisely critical and some advantage appears to accrue to the incorporation of even as little as 25% of the product on the weight of the asphalt. Generally speaking, however, it is desirable to employ between 25% and 130% on the weight of the asphalt, about three parts of condensation product to four parts of asphalt being optimum in a great many compositions.

The asphalt to be employed should be of amode'rately soft type, with a softening point of 180 to 220 F., the optimum softening point lying in the range of 180 to 190 F. Excellent results have been obtained with asphalt having a penetration at 77 F. of 70 to 90 tenths of a mm./5 secs. at a loading of 100 grams. A preferred form of asphalt is a 101W aromatic content, airlown asphalt derived from petroleum and having a softening point between 180 and 190 and a penetration at 77 F. of from 70 to 90 tenths of a mm./5 secs. at a loading of 100 grams.

Although the figures are not particularly significant,

since proportions of the asphalt in the product depend upon the nature of the 'oxidizer employed as well as tates atent ice Patented Sept. 19, 1961 upon the proportions of the other ingredients, propellants prepared in accordance with my invention usually contain from 8 to 12% by weight of asphalt.

The addition of the condensation product of sebacic acid with a polyhydric alcohol obtained by reacting these two substances has the effect of inhibiting the cracking of the propellant grain. Preferably, the polyhydric alcohol employed in compounding the condensation product should be a liquid dihydric or trihydric alcohol such as glycerol or glycol. However, other polyhydric alcohols, including the sugars, may be employed. I have found that the optimum results may be secured if a long chain condensation product of the dibasic acid and the polyhydric alcohol is used which is incompletely reacted when it is incorporated into the propellant mixture. Such a condensation product usually has a tendency of reacting further; but such further reaction may be prevented in part or totally by the incorporation of a reaction inhibitor as described in detail hereafter.

A preferred example of a high molecular weight condensation product, for use in the practice of this invention, is the compound obtained when glycerol and sebacic acid are condensed together. A portion of the ingredients forming the condensation product remain unreacted and unless prevented from reacting further thesesubstances would tend to increase the length of the chain of the condensation product. To avoid this, I incorporate into the condensation product a reaction inhibitor to delay or prevent such reaction. For example, small proportions of the fatty acids, non-drying fats and vegetable oils serve to prevent this further reaction. Castor oil is the preferred example of vegetable oil and is particularly suitable for preventing the further increase in the size of the glyceryl sebacate condensation product molecule. The proportion of the reaction inhibitor to be employed depends upon the stability of the condensation product and other factors. Usually, however, a small amount of the polymerization inhibitor is required, this ordinarily being less than 35% by weight based on the weight of the condensation product.

I have also discovered that undesirable flow and sagging at high temperatures may be inhibited and in large part prevented in plastic, aliphatic base propellants containing as oxidizer an intimately incorporated finely divided inorganic salt, as mentioned above, by incorporating into the charge substantial proportions of waxes preferably having melting points above the upper limit of the. temperature range to which the propellant may be subjected. Preferred waxes have melting and softening points well above the upper limit of the temperature range to which the propellant may be subjected prior to combustion but below the casting temperature. In other words, the wax should be such that it does not begin to sag or flow appreciably until the casting temperature of the propellant mixture is approached.

The use of such a wax in the propellant composition tends to raise the temperature at which the entire propellant mixture will sag or begin to flow, but, however, without making it appreciably harder at the lower temperatures, so that the wax aids the condensation product in preventing sagging without increasing the tendency of the charge to crack. Almost any of the waxes having melting points at temperatures in excess of F. are satisfactory. Thus, synthetic waxes such as cetyl acetamide are good. This particular wax is liquid at the mixing and casting temperatures of asphaltic propellants, i.e., about 260 F. Petroleum mierocrystalline waxes with melting points of at least 180 F. and paraffinic waxes with melting points above 160 F. may also be employed.

In general, the proportions of wax to be employed, range from to 125% on the Weight of the condensation product. In most charges, the wax should be 2 /z% to 4% of the total weight, 3% of the total weight being optimum in many instances.

In some cases the wax may tend to harden the charge slightly at lower temperatures, and any such tendency may be successfully combatted by incorporating a softening agent which is a high-boiling solvent for the wax that is also compatible with the other constituents of the charge. For example, the softening agent may be a solvent in the form of a liquid organic ester having a boiling point above the casting temperature, say 260 F., and with a low vapor pressure at that temperature. An excellent example of a suitable solvent is dibutyl sebacate.

The proportions of softening agent to be employed are not critical, although in general if the softening agent is employed at all, it may be employed in proportions ranging from 50% to 240% by weight of the wax in the charge. In many instances, this will result in a wax content of 2% to 6% on the weight of the total charge. In the case of dibutyl sebacate, it is desirable to use from 50% to 140% on the weight of the wax, which in most cases will result in from 2% to 3 /z% by weight of this constituent in the charge as a Whole, a satisfactory operating figure being approximately 3%. Since some of the softening agent is lost due to evaporation in compounding and casting operations, actual proportions in the charge may be somewhat lower than indicated.

Various types of inorganic salts of strong oxidizing character are useful for mixing with asphalt to form propellants, including, as indicated above, chlorates, perchlorates, chromates, permanganates, etc. The percentage of ammonium perchlorate, for example, can range from about 65% by weight to about 90% by weight of the total weight of the propellant composition. The condensation products of sebacic acid and polyhydric alcohol, either with or without the waxes, as described above, would be useful in the reduction of the cracking tendency of such propellants as would otherwise tend to crack. Although, the use of the condensation product in my novel mixture would produce more or less benefit in connection with all propellants of this type which would otherwise be unduly brittle, nevertheless, my novel mixture of the condensation product, particularly with the waxes, is especially significant and effective when ammonium perchlorate is used as the particular oxidizing substance.

Ammonium perchlorate is an especially desirable oxidizer because it has a number of advantages which make it outstanding relative to other oxidizing materials. One of its most important advantages is that it results in combustion products which are practically smokeless. Moreover the burning rate is not nearly as much affected by the ambient temperature as when other oxidizers are used in the asphaltic propellant. A factor which has heretofore made it difiicult to use ammonium perchlorate in the propellant, in spite of these obvious outstanding advantages, is that its specific weight is so low that an unduly large bulk of it must be mixed with asphalt in order to make available enough oxygen to burn the fuel. This great preponderance of the ammonium perchlorate in the bulk of the propellant has made a brittle and friable propellant mass which is uncohesive and does not present a uniform, compact, burning surface and mass which is highly desirable in a rocket motor for the purpose of a uniform burning rate.

When the condensation product of sebacic acid and polyhydric alcohol, preferably with the wax addition, as described above, is mixed with such an asphalt and ammonium perchlorate propellant, the unique effect is produced that the propellant mass becomes coherent and compact presenting the desired type of propellant burning surface which will burn uniformly in a rocket motor chamber. Furthermore, the mass is exceptionally resistant to cracking over a wide range of ambient temperature even down to such low temperatures as -40 F. Moreover, even while obtaining the outstanding advantage of being resistant to cracking down to such a low temperature, I have found that it is at the same time highly resistant to sagging which so often results from undue softening when other kinds of propellants are used; and this sagging resistance is effective up to ambient temperatures even as high as 160 F. Another great advantage is that a propellant containing ammonium perchlorate is far more easily ignited over a wider range of ambient temperatures than propellants using other oxidizers.

In compounding the propellant employing all of the types of constituents described above, I prefer to incorporate the wax in the melted asphalt and then add to this a mixture of the condensation product with the reaction inhibitor. The solvent or softening agent is then added because it is the most volatile of the ingredients. Then the oxidizer is stirred in. For example, cetyl acetamide may be stirred into the melted asphalt, followed by a fatty acid-modified glyceryl sebacate or glycol sebacate, dibutyl sebacate being added next, and finally ammonium perchlorate (when ammonium perchlorate, the preferred oxidizer, is used). The compounded ingredients are then mixed at approximately 260 F. in the liquid condition. After adequate mixing, the mixture is cast into a propellant stick and permitted to solidify. The hardened stick preferably is dipped in asphalt and wrapped with a liner of plastic or of tape, paper and asphalt.

A preferred propellant composition of my invention is compounded according to the following proportions by weight:

a Percent Asphalt 8-12 Castor oil modified glyceryl sebacate 7-11 Dibutyl sebacate 241 Cetyl acetamide (commercial grade Acrawax B or C) W. 2 /2-4 Ammonium perchlorate 75 Another example for a suitable smokeless propellant The expression castor oil modified glyceryl sebacate as used herein means the modification of the alkyd resin by preventing the molecule from becoming larger due to continued reaction between the unreacted components and the condensation product.

One preferred asphalt for use in compounding this propellant is an asphalt having a penetration at 77 F. of from 70 to tenths of a mm./5 sec., at a loading of grams and which has a softening point of to F.

Although the other waxes noted above may be used in place of cetyl acetamide, a special advantage accrues to the use of one commercial grade of the latter, this grade being modified with stearic acid, so that its use prevents the undesired further reaction of the glyceryl sebacate. This is due to the presence of the free fatty acid.

The use of the propellant of this invention in a jet propelled device is illustrated in the accompanying drawing which shows a jet motor partly in cross section. This comprises a cylindrical body 1 which is provided with a removable head 2 and which is sealed at the other end by a plate permanently attached to the end of the cylinder by some suitable manner. This 'head 2 may be preferably attached by threads to the cylindrical body 1. A nozzle 3 fits into the head as illustrated and 'is held in position by a collar 4. This collar is secured to the head 2 by bolts 5. Ignitor squib 6 is screwed into the head and enables the operator to ignite the'main charge when an electric current is .permitted to flow through the terminals '7 of squib 6. The inside of cylindrical body 1 is provided with a liner 8 which may preferably be made of cloth, asphalt and paper, plastic or any other suitable material. The main charge 9 which is compounded according to my present invention is 'then placed in liner 8 and when inserted in the motor fills "the cylinder to within a short distance of the nozzle. The propellant is preferably cast outside the motor, enclosed in a suitable liner and introduced into the motor as a cartridge. When used in this manner the propellant does not need to be bound to the walls of the motor as is the customary procedure when the propellant is cast directly in the motor.

My propellant, when made with my preferred oxidizer, ammonium perchlorate, possesses the advantage of having suitable physical properties over a temperature range extending between 40 F. and +l60 F., and has a burning rate at chamber pressure of 1000 lbs./sq./in. of approximately A1 in./sec.

An outstanding advantage of such propellant is the tendency of the propellant to close up any irregularities or cracks which are present at the time it starts to burn under pressure. Moreover, the burning rate is not affected by the ambient temperature changes such as is the case with many other types of asphaltic propellants.

Lastly, I have found that with this propellant it is possible to operate a jet motor at a low combustion rate even though the pressure in the chamber of the motor is maintained at low value, say as low as 750 lbs./sq./in. This contrasts favorably with the difficulty of maintaining continuous low burning rates at pressure under 1000 lbs./sq./in. with other types of asphaltic propellants.

I claim:

1. An asphalt base propellant containing from about 8% to about 12% by weight, based on the weight of the total propellant, of an asphalt having a softening point between 180 F. and 190 F. and a penetration at 77 F. of from 70 to 90 tenths of a rnm./5 sec. loading under 100 grams; from about 25% to about 130% by weight, based on the weight of the asphalt, of an alkyd resin comprising a condensation product of sebacic acid with a polyhydric alcohol selected from the group consisting of glycol and glycerol; from a trace to about 35 by weight of castor oil, based on the weight of the condensation product; from about 20% to about 125% by weight, based on the weight of the condensation product, of a wax having a melting point between 160 F. and 260 F.; and the balance being an inorganic oxidizing salt.

2. An asphalt base propellant containing from about 8% to about 12% by weight, based on the weight of the total propellant, of an asphalt having a softening point between 180 F. to 190 F. and a penetration at 77 F. of from 70 to 90 tenths of a mm./5 sec. loading under 100 grams; from about 25% to about 130% by weight based on the weight of the asphalt, of glycol sebacate alkyd resin; from a trace to about 35% by weight, based on the glycol sebacate resin, of castor oil; from about 20% to about 125% by weight, based on the weight of the condensation product, of a wax having a melting point between 160 F. and 260 F.; and the balance being an inorganic oxidizing salt.

3. An asphalt base propellant containing from about 8% to about 12% by weight, based on the weight of the total propellant, of an asphalt having a softening point between 180 F. and 190 F. and a penetration at 77 F. of from 70 to 90 tenths of a mm./ 5 sec. loading under 100 grams; from about 25% to about 130% by weight, based on the weight'of the asphalt, of glyceryl sebacate alkyd Iresin; from atrace to about 35% by weight, based on the weight of the .glyceryl sebacate, of castor oil; from about 20% to about 125% by weight, based on the weight of the condensation product, of a wax :having a melting point between 160 F. and 260 F.; and the balance being an inorganicoxidizing salt.

4. An asphalt base propellant containing from about 8% to about 12% by weight, based on the Weight of the total propellant, of an asphalt having a softening point between 180 F. and 190 F. and a penetration at 77 -F.-of irom 70 to '90 tenths "of a mm./ 5 sec. loading under 100 grams; and from about 25% to about 130% by weight, based on the weight of the asphalt of castor oil modified glyceryl alkyd resin sebacate; from about 20% to about 125% by weight, based on the weight of the glyceryl sebacate of a wax having a melting point between 160 F. and 260 F.; and the balance being an inorganic oxidizing salt.

5. An asphalt base propellant containing from about 8% to about 12% by weight, based on the weight of the total propellant, of an asphalt having a softening point between 180 F. and 190 F. and a penetration at 77 F. of from 70 to tenths of a mm./5 sec. loading under grams; from about 25% to about 130% by weight, based on the weight of the asphalt of castor oil modified glyceryl sebacate alkyd resin; from about 20% to about by weight, based on the weight of the glyceryl sebacate, of cetyl acetamid; and the balance being an inorganic oxidizing salt.

6. An asphalt base propellant containing from about 8% to about 12% by weight, based on the weight of the total propellant, of an asphalt having a softening point between 180 to 190 F. and a penetration at 77 F. of from 70 to 90 tenths of a mm./ 5 sec. loading under 100 grams; from about 25% to about by weight based on the weight of the asphalt; of an alkyd resin comprising a condensation product of sebacic acid with a polyhydric alcohol selected from the group consisting of glycol and glycerol; from a trace to about 35% by weight, based on the weight of the condensation product, of castor oil; from about 20% to about 125 by weight, based on the weight of the condensation product, of a wax having a melting point between F. and 260 F.; from about 50% to about 240% by weight, based on the weight of the wax, of dibutyl sebacate; and the balance being an inorganic oxiding salt.

7. An asphalt base propellant containing from about 8% to about 12% by weight, based on the weight of the total propellant, of an asphalt having a softening point between 180 F. and F. and a penetration at 77 F. of from 70 to 90 tenths of a mm./ 5 sec. loading under 100 grams, from about 25 to about 130% by weight, based on the weight of the asphalt, of an alkyd resin comprising a condensation product of sebacic acid with a polyhydric alcohol selected from the group consisting of glycol and glycerol; from a trace to about 35% byweight, based on the weight of the condensation product of castor oil; from about 20% to about 125 by weight, based on the weight of the condensation product, of a wax having a melting point between 160 F. and 260 F.; and from about 65% to about 90% by weight, based on the weight of the total propellant, of ammonium perchlorate.

8. A propellant composition comprising from between about 8% to about 12% by weightbased on the weight of the total propellant of asphalt having a softening point of 180 to 190 F. and a penetration of 70 to 90 tenths of a mm./5 sec. loading under 100 grams; from about 7% to about 11% by weight, based on the weight of the total propellant, of a castor oil modified glyceryl sebacate alkyd resin; from about 2% by weight to about 2 /2 by weight, based on the total weight of the propellant, of dibutyl sebacate; from about 2 /2% to about 4% by weight, based on the weight of the propellant, of cetyl acetamid; and about 75% by weight, based on the weight of the total propellant of ammonium perchlorate.

9. An asphalt base propellant composition comprising about 10 /2% by weight, based on the weight of the total propellant, of asphalt having a softening point of 180 F. to 190 F.; about 3% by weight, based on the weight of the propellant, of cetyl acetarnid; about 8% by weight, based on the weight of the propellant, of castor oil modified glyceryl sebacate alkyd resin; about 3 /2% by weight, based on the total weight of the propellant, of dibutyl sebacate; and about 75% by Weight based on the total weight of the propellant of ammonium perchlorate.

References Cited in the file of this patent UNITED STATES PATENTS 

1. AN ASPHALT BASE PROPELLANT CONTAINING FROM ABOUT 8% TO ABOUT 12% BY WEIGHT, BASED ON THE WEIGHT OF THE TOTAL PROPELLANT, OF AN ASPHALT HAVING A SOFTENING POINT BETWEEN 180*F. AND 190*F AND A PENETRATION AT 77*F. OF FROM 70 TO 90 TENTHS OF A MM./5SEC. LOADING UNDER 100 GRAMS, FROM ABOUT 25% TO ABOUT 130% BY WEIGHT, BASED ON THE WEIGHT OF THE ASPHALT, OF AN ALKYD RESIN COMPRISING A CONDENSATION PRODUCT OF SEBACIC ACID WITH A POLYHYDRIC ALCOHOL SELECTED FROM THE GROUP CONSISTING OF HLYCOL AND GLYCEROL, FROM A TRACE TO ABOUT 35% BY WEIGHT, BASED ON THE WEIGHT OF THE CONDENSATION PRODUCT, OF A WAX HAVING A MELTING POINT BETWEEN 160* F. AND 260*F., AND THE BALANCE BEING AN INORGANIC OXIDIZING SALT. 